There are many situations where it is desired to identify chemical compounds in a sample. Such samples may be taken directly from the environment or they may be provided by front end specialized devices to separate or prepare compounds before analysis. Furthermore, recent events have seen members of the general public exposed to dangerous chemicals in situations where previously no thought was given to such exposure. There exists, therefore, a demand for low cost, accurate, easy to deploy and use, reliable devices capable of identifying the chemical content of a sample.
One class of known chemical analysis instruments is referred to as mass spectrometers. Mass spectrometers are generally recognized as being the most accurate type of detectors for compound identification, given that they can generate a fingerprint pattern for even fragment ions. However, mass spectrometers are quite expensive and large and are relatively difficult to deploy in the field. Mass spectrometers also suffer from other shortcomings such as the need to operate at low pressures, resulting in complex support systems. These systems also require a highly trained user to tend to operations and interpret results.
Another class of known chemical analysis instruments enable used of atmospheric-pressure chemical ionization. Ion analysis is based on the recognition that ion species have different ion mobility characteristics under different electric field conditions at elevated pressure conditions including atmospheric pressure. Practices of the concept include time-of-flight Ion Mobility Spectrometry (IMS) and differential mobility spectrometry (DMS), the latter also sometimes referred to as field asymmetric ion mobility spectrometry (FAIMS). These systems enable chemical species identification at atmospheric pressure, preferably based on dry and clean gas samples.
In a conventional time-of-flight IMS device (sometimes referred to as TOF-IMS), a propelling DC field gradient and a counter gas flow are set and an ionized sample is released into the field which flows to a collector electrode. Ion species are identified based on the DC field strength and time of flight of the ions to the collector. The electric field is weak where ion mobility is constant.
DMS systems identify ion species by mobility behavior in a high asymmetric RF field, where ions flow in a carrier gas and are shifted in their path by an electric field. The conventional DMS operates with at a selected RF field at Vmax and species detections are correlated with a pre-set, or scanned, DC compensation voltage (Vc). Species are identified based upon correlation of Vmax and Vc with historical detect data. It is well-known that for a given ion species in a sample, as the amplitude of the asymmetric RF voltage (at Vmax) changes, the amplitude of the DC compensation voltage (Vc) required for passage of that species through the filter field will also change. The amount of compensation depends upon species characteristics.
A typical DMS device includes a pair of opposed filter electrodes defining an analytical gap between them in a flow path (also known as a drift tube). Ions flow into the analytical gap. An asymmetric RF field (sometimes referred to as a filter field, a dispersion field or a separation field) is generated between the electrodes transverse to the carrier gas/ion flow in the gap. Field strength, E, varies as the applied RF voltage (sometimes referred to as dispersion or separation voltage, or Vrf) and size of the gap between the electrodes. Such systems operate at atmospheric pressure.
Ions are displaced transversely by the RF field, with a given species being displaced a characteristic amount toward the electrodes per cycle. DC compensation (Vc) is applied to the electrodes along with Vrf to compensate the displacement of a particular species. Now the applied compensation will offset transverse displacement generated by the applied Vrf for that particular ion species. The result is zero or near-zero net transverse displacement of that species, which enables that species to pass through the filter for detection. All other ions undergo a net displacement toward the filter electrodes and will eventually undergo collisional neutralization on one of the electrodes.
If the compensation voltage is scanned for a given RF field, a complete spectrum of ion species in the sample can be produced. The recorded image of this spectral scan is sometimes referred to as a “mobility scan”, as an “ionogram”, or as “DMS spectra”. The time required to complete a scan is system dependent. Relatively speaking, a prior art IMS scan might take on the order of a second to complete while and a prior art DMS might take on the order of 10 seconds to complete.
DMS operates based on the fact that an ion species will have an identifying property of high and low field mobility in the analytical RF field. Thus DMS detects differences in an ion's mobility between high and low field conditions and classifies the ions according to these differences. These differences reflect ion properties such as charge, size, and mass as well as the collision frequency and energy obtained by ions between collisions and therefore enable identification of ions by species.
Illustrative examples of mobility scans based on the output from a DMS device are shown in FIG. 1A and FIG. 1B. As shown in FIG. 1A, a single compound, acetone, was submitted to the DMS analyzer. The illustrated plot is typical of the observed response of the DMS device, with detected acetone ions in this example forming a peak intensity at a compensation voltage of about −1.5 volts. This is useful information, such that future detections of a peak at this compensation in this device is indicative of detection of acetone.
In FIG. 1B, the analyzed sample consisted of acetone and an isomer of xylene (o-xylene). The acetone peak appears at about −2.5 volts while o-xylene appears at about −4 volts. Data representing these detection peaks can be compared against stored data for known compounds for this device and the applied RF field and compensation, and identification is made based upon a data match. FIG. 1B demonstrates unique detection peaks according to ion mobility characteristics for different ion species in the sample under test, i.e., o-xylene and acetone.
Various chemical species in a sample can be identified according to the conventional DMS process. However, accurate identification of several species in a sample whose detection spectra overlap is difficult. This is in part due to the fact that DMS detection peaks are relatively broad compared to a mass spectrometer, so overlap is more likely than with a mass spectrometer. In fact, where several ion species exhibit similar behavior in the DMS filter field their associated DC compensation will be very close, and so their detection spectra (detection peaks) will present as overlapped.
This “overlap” of detection peaks interferes with species identification. But discrimination between overlapping spectra is not easily achieved and similar species are not so easily separated.
Furthermore, false negative detections are dangerous when dangerous compounds are at issue, while false positives can reduce trust in a detection system. Therefore improved spectrometer performance is an important goal of the present invention.
It is therefore an object of the present invention to provide a fast and simple system, whether method or apparatus, capable of a high degree of species discrimination and accurate species identification for chemical analysis.